Optical devices are particularly suitable for inspecting the surface of wafers. Examination of the surface can be accomplished, for example, as is known from EP 455 857, by evaluating rays that are reflected from the surface of the wafer.
Also known are optical apparatuses that, by image recognition, allow the detection of a great variety of features on the surface of a wafer or a semiconductor substrate. In this context the wafer is usually illuminated in bright-field fashion and scanned with a camera, for example a matrix camera or linear camera.
It is furthermore known from U.S. Pat. No. 6,587,193 to examine the surface of a wafer, an illumination being selected that scans the wafer in the form of a line. The illuminating line is guided over the surface of the wafer so that a two-dimensional image can be produced.
US 2003/0202178 A1 furthermore discloses a method and an apparatus for examining a wafer. Here an illumination is irradiated onto the wafer so that an edge of the wafer is struck. The edge of the wafer can thus be sensed and processed by an image processing unit. Defects of the wafer can be ascertained by a comparison of the ascertained edge image with a stored comparison image.
The known systems for inspecting a wafer are designed exclusively for incident-light inspection. The reason for this is principally that silicon wafers are opaque in the region of visible, ultraviolet, and deep ultraviolet light wavelengths. Silicon becomes transparent only at a wavelength above 1000 nm. In these wavelength regions the possibility then presents itself of being able to inspect features below the surface of the wafer, or to observe features on the front side of the wafer through the back side.
For the transmitted-light examination of wafers, however, the known illumination concepts of transmitted-light microscopy require a transmitted-light illuminating optical system below the microscope and a transmitted-light-capable microscope stage, which is not implemented in presently known examination systems. An expansion of the known examination systems to include transmitted-light inspection therefore requires a completely new design. In particular, such systems would need to be equipped with a wafer microscope stage suitable for transmitted light, which has, in contrast to the incident-light wafer microscope stages used hitherto, an unobstructed passthrough opening for the transmitted-light illumination over the entire wafer diameter. Considerable design effort for integrating a transmitted-light illumination system into the wafer inspection device is also necessary.